Lithium metal phosphate compounds, in particular lithium transition metal phosphate compounds, have recently become widely used as cathode and anode materials in secondary ion batteries (U.S. Pat. No. 5,910,382, WO 02/099913). In addition to solid-state syntheses for such lithium transition metal phosphates, wet-chemical methods are also used to manufacture such compounds, such as are described for example in DE 10353266 A1 or in WO 02/083555. The particle morphology of the produced lithium transition metal phosphate can also be particularly well controlled in such wet-chemical methods, for example.
In these wet-chemical methods, lithium hydroxide, which is brought together with an acid solution usually containing phosphoric acid, and with at least one transition metal sulphate, in one of the synthesis steps, is typically used as lithium starting compound. However, a particular problem with the manufacture of lithium transition metal phosphates using such wet-chemical methods is the high loss of lithium ions in the so-called waste brine, the renewed use of which in typical industrial cyclic processes, in particular when processing and recovering the starting materials, is made difficult by high levels of foreign ions, especially by sulphate.
Hitherto, only purifying methods that are extremely costly in process engineering terms have been known for reprocessing the lithium-containing waste brine. A further possibility is described for example in DE 10 2007 033460, wherein barium sulphate is precipitated by adding barium hydroxide and LiOH remains in solution. However, a disadvantage of this method is the large amount of barium sulphate produced, obtainable only with difficulty for further industrial use in acceptable so-called blanc fixe types.